The Severins Bridge in Cologne, completed in 1959, is a cable-stayed bridge with an asymmetrical A-pylon, a maximum span length of 302 m and a total length of 691 m. The superstructure consists of two box girders connected by an orthotropic deck slab. It carries two lanes of traffic in either direction, two KVB city rail tracks and pedestrian and cycle paths over the Rhine River.

As part of a structural analysis of the river bridge, which our engineers were commissioned to carry out by the city of Cologne, various target load levels combined with several load models for the city rail were analysed. The longitudinal ribs were found to buckle in some of the load cases.

For the subsequently developed strengthening measures targeting this deficit, the geometric boundary conditions needed to be taken into account. The long transport distances, the limited space available inside the box girders (3.20 m × 4.56 m) and pylon sections (3.58 m × 3.58 m) due to the longitudinal and transverse stiffeners and other installations were to be considered, as well as the imperfections of the longitudinal stiffeners determined by laser 3D scan. The constraints resulting from these geometrical conditions informed the design and tender documents. A construction boasting additional access facilities for staff and material transport was chosen. Its elements could be transported and installed by hand inside the structure and attached by variable screw connections. Nearly all required drilling was done on site, with minimum preparation in the factory.

Galvanisation was chosen as a durable and economic corrosion protection for the strengthening structure, ensuring that no work safety problems would occur during maintenance or in-situ coating.